Process of Forming a Microphone Using Support Member

ABSTRACT

A method of forming a microphone forms a backplate, and a flexible diaphragm on at least a portion of a wet etch removable sacrificial layer. The method adds a wet etch resistant material, where a portion of the wet etch resistant material is positioned between the diaphragm and the backplate to support the diaphragm. Some of the wet etch resistant material is not positioned between the diaphragm and backplate. The method then removes the sacrificial material before removing any of the wet etch resistant material added during the prior noted act of adding. The wet etch resistant material then is removed substantially in its entirety after removing at least part of the sacrificial material.

PRIORITY

This patent application claims priority from provisional U.S. patentapplication No. 60/754,984, filed Dec. 29, 2005, entitled, “ PROCESS OFFORMING A MICROPHONE USING SUPPORT MEMBER,” attorney docket number2550/A84, and naming Jason Weigold as inventor, the disclosure of whichis incorporated herein, in its entirety, by reference, and is acontinuation of U.S. patent application Ser. No. 12/244,840, filed Oct.3, 2008, entitled, “PROCESS OF FORMING A MICROPHONE USING SUPPORT MEMBER,” attorney docket number 2550/C18, and naming Jason Weigold asinventor, the disclosure of which is incorporated herein, in itsentirety, by reference, and also claims priority from U.S. patentapplication Ser. No. 11/613,003, filed Dec. 19, 2006, entitled, “PROCESSOF FORMING A MICROPHONE USING SUPPORT MEMBER,” attorney docket number2550/B39, and naming Jason Weigold as inventor, the disclosure of whichis incorporated herein, in its entirety, by reference.

This patent application also claims priority from U.S. patentapplication Ser. No. 11/113,925, filed Apr. 25, 2005, now U.S. Pat. No.7,825,484, issued Nov. 2, 2010 and entitled, “ MICROMACHINED MICROPHONEAND MULTISENSOR AND METHOD FOR PRODUCING SAME ,” attorney docket number2550/A47, and naming John R. Martin, Timothy J. Brosnihan, Craig Core,Thomas Kieran Nunan, Jason Weigold, and Zin Zhang as inventors, thedisclosure of which is incorporated herein, in its entirety, byreference.

FIELD OF THE INVENTION

The invention generally relates to microphones and, more particularly,the invention relates to methods for forming microphones.

BACKGROUND OF THE INVENTION

Some processes form microphones, such as MEMS microphones, by depositinga diaphragm material onto a sacrificial material that ultimately isremoved by wet etching processes. Problems arise, however, when thesurface tension of the liquid etchant causes the diaphragm to stick toits corresponding backplate. See U.S. Pat. No. 5,314,572 for an exampleof one method discussed for use with inertial sensors.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method of forming amicrophone forms a backplate, and a flexible diaphragm on at least aportion of a wet etch removable sacrificial layer. The method adds a wetetch resistant material, where a portion of the wet etch resistantmaterial is positioned between the diaphragm and the backplate tosupport the diaphragm. Some of the wet etch resistant material is notpositioned between the diaphragm and backplate. The method then removesthe sacrificial material before removing any of the wet etch resistantmaterial added during the prior noted act of adding. The wet etchresistant material then is removed substantially in its entirety afterremoving at least part of the sacrificial material.

Some embodiments release the diaphragm, which involves removing thesacrificial material and removing the wet etch resistant material. Insome embodiments, the wet etch resistant material comprises aphotoresist material. Moreover, the wet etch resistant material may beremoved by application of a dry etch. It should be noted that the act offorming a flexible diaphragm may include a number of different things.For example, it may involved depositing a material on a sacrificiallayer, or simply providing a silicon wafer on an insulator, such as thetop layer of a silicon-on-insulator (“SOI”) wafer.

In illustrative embodiments, the wet etch resistant material is asingle, substantially contiguous apparatus, which is substantiallycompletely removed by the process in a single act. Moreover, the wetetch resistant material may be unpatterned after the portion of wet etchresistant material is positioned between the diaphragm and thebackplate.

The backplate may be formed from a number of different types of wafers,such as a part of a SOI wafer. As a further example, the sacrificialmaterial may be formed from polysilicon or oxide.

Various processes may be executed to complete the microphone. Forexample, the method may form a first hole through the diaphragm, and asecond hole through the sacrificial material. The second holeeffectively produces a channel between a bottom surface of the diaphragmand a top surface of the backplate. To support the diaphragm, theportion of wet etch resistant material substantially fills the channel.In some embodiments, the wet etch resistant material substantiallycompletely fills the first hole even when the sacrificial material isremoved.

In accordance with another embodiment of the invention, a method offorming a MEMS microphone provides an SOI wafer having a top layer,forms a sacrificial material on the top layer of the SOI wafer, andforms a diaphragm on the sacrificial material. The method also forms ahole through the diaphragm, and a channel through the sacrificialmaterial. The hole and channel are in fluid communication, and thechannel exposes a bottom surface of the diaphragm and a top surface ofthe top layer of the SOI wafer. The method then adds a wet etchresistant material having a first portion within the channel and asecond contiguous portion that is external to channel and substantiallycompletely fills the hole through the diaphragm. At least a portion ofthe sacrificial material is removed before removing any of the wet etchresistant material.

The method eventually may release the diaphragm. To that end, the wetetch resistant material illustratively is removed after removing atleast part of the sacrificial material. In some embodiments, the methodremoves a portion of the bottom and middle SOI layers to form a backsidecavity. The method also may form a backplate hole through the top layerof the SOI wafer. Illustrative embodiments ensure that the wet etchresistant material is unpatterned after the wet etch resistant materialis added to the channel.

In accordance with another embodiment of the invention, a MEMSmicrophone apparatus has a backplate, a diaphragm having a through-holeand top and bottom sides, and a substantially contiguous, unpatterned,wet etch resistant material, between the backplate and diaphragm. Thematerial supports the diaphragm by being in contact with both the topand bottom sides of the diaphragm. In addition, the apparatus has an airspace between portions of the diaphragm and the backplate.

In accordance with other embodiments of the invention, a method offorming a MEMS microphone provides a diaphragm layer on a sacrificiallayer, where the sacrificial layer is between the diaphragm layer and asubstrate. Next, the method forms a wet etch resistant material betweenthe substrate and the diaphragm, and forms a backside cavity. The methodthen applies a wet etch material to the sacrificial material through thebackside cavity to remove at least a portion of the sacrificial layer.At least a portion of the wet etch resistant material supports thediaphragm layer after applying the wet etch material.

In some embodiments, the diaphragm layer, sacrificial layer andsubstrate are respective layers of a single SOI wafer. Moreover, the actof providing may include depositing a material on the sacrificial layerto form the diaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages of the invention will be appreciated more fullyfrom the following further description thereof with reference to theaccompanying drawings wherein:

FIG. 1A schematically shows a perspective view of an unpackagedmicrophone that may be fabricated in accordance with illustrativeembodiments of the invention.

FIG. 1B schematically shows a cross-sectional view of the microphoneshown in FIG. 1A.

FIGS. 2A and 2B show a process of forming the microphone shown in FIGS.1A and 1B in accordance with illustrative embodiments of the invention.

FIG. 3A schematically shows a view of the microphone during a stage ofthe method shown in FIG. 2A (steps 200-208).

FIG. 3B schematically shows a view of the microphone during a stage ofthe method shown in FIG. 2A (step 214).

FIG. 3C schematically shows a view of the microphone during a stage ofthe method shown in FIG. 2A (step 216).

FIG. 3D schematically shows a view of the microphone during a stage ofthe method shown in FIG. 2A (step 218).

FIG. 3E schematically shows a view of the microphone during a stage ofthe method shown in FIG. 2A (step 220).

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, a method of forming a microphone uses aliquid etch resistant material to prevent its diaphragm from adhering orotherwise sticking to its backplate during a liquid/wet etch step. Tothat end, the illustrative method forms a wet etch resistant materialbetween the diaphragm and backplate, thus supporting the diaphragmduring a wet etch step. Various embodiments eliminate patterning stepsby ensuring that the liquid etch material is added and removed inrelatively few steps (e.g., one or two steps for adding, and a singlestep for removing-no specialized patterning processes for the wet etchresistant material are necessary). Details of illustrative embodimentsare discussed below.

FIG. 1A schematically shows a top, perspective view of a microphone 10(also referred to as a “microphone chip 10”) that may be fabricated inaccordance with illustrative embodiments of the invention. FIG. 1Bschematically shows a cross-section of the same microphone 10 acrossline B-B of FIG. 1A.

Among other things, the microphone 10 includes a static backplate 12that supports and forms a capacitor with a flexible diaphragm 14. Inillustrative embodiments, the backplate 12 is formed from single crystalsilicon (e.g., the top layer of a silicon-on-insulator wafer), while thediaphragm 14 is formed from deposited polysilicon. Other embodiments,however, use other types of materials to form the backplate 12 and thediaphragm 14. For example, a single crystal silicon bulk wafer, or somedeposited material may form the backplate 12. In a similar manner, asingle crystal silicon bulk wafer, part of an silicon-on-insulatorwafer, or some other deposited material may form the diaphragm 14. Tofacilitate operation, the backplate 12 has a plurality of through-holes16 that lead to a backside cavity 18.

Springs 19 movably connect the diaphragm 14 to the static portion of themicrophone 10, which includes the substrate. Audio signals cause thediaphragm 14 to vibrate, thus producing a changing capacitance. On-chipor off-chip circuitry (not shown) receive (via contacts 15) and convertthis changing capacitance into electrical signals that can be furtherprocessed. It should be noted that discussion of the specific microphone10 shown in FIGS. 1A and 1B is for illustrative purposes only. Othermicrophone configurations thus may be used with illustrative embodimentsof the invention.

FIGS. 2A and 2B show a process of forming the microphone 10 shown inFIGS. 1A and 1B in accordance with illustrative embodiments of theinvention. The remaining figures illustrate various steps of thisprocess. It should be noted that this process does not describe allsteps required for forming the microphone 10. Instead, it shows variousrelevant steps for forming the microphone 10. Accordingly, some stepsare not discussed.

The process begins at step 200, which etches trenches 20 in the toplayer of a silicon-on-insulator wafer (“SOI wafer”). FIG. 3Aschematically shows an intermediate apparatus 22 illustrating this step,as well as steps 202-208. Next, the process adds sacrificial oxide 24 tothe walls of the trenches 20 and along at least a portion of the topsurface of the top layer of the SOI wafer (step 202). Among other ways,this oxide 24 may be grown or deposited. Step 202 continues by addingsacrificial polysilicon 26 to the oxide lined trenches 20 and top-sideoxide 24.

After adding the sacrificial polysilicon 26, the process etches a hole28 into the sacrificial polysilicon 26 (step 204). The process thencontinues to step 206, which adds more oxide 24 to substantiallyencapsulate the sacrificial polysilicon 26. In a manner similar to othersteps that add oxide 24, this oxide 24 essentially integrates with otheroxides in which it comes into contact. Step 206 continues by adding anadditional polysilicon layer that ultimately forms the diaphragm 14.

Nitride 30 for passivation and metal 32 for electrical connectivity alsoare added. For example, deposited metal may be patterned to form a firstelectrode for placing electrical charge on the diaphragm 14, anotherelectrode for placing electrical charge on the backplate 12, and bondpads 15 for providing additional electrical connections. There may beelectrical connections (not shown) between bond pads and the electrodes.

The process then both exposes the diaphragm 14, and etches holes 34through the diaphragm 14 (step 208). As discussed below in greaterdetail, one of these holes (“diaphragm hole 34A”) ultimately assists informing a pedestal 42 that, for a limited time during this process,supports the diaphragm 14. A photoresist layer 36 then is added,completely covering the diaphragm 14 (step 210). This photoresist layer36 serves the function of an etch mask.

After adding the photoresist 36, the process exposes the diaphragm hole34A (step 212, FIG. 3B). To that end, the process forms a hole (“resisthole 38”) through the photoresist 36 by exposing that selected portionto light. This resist hole 38 illustratively has a larger inner diameterthan that of the diaphragm hole 34A.

After forming the resist hole 38, the process forms a hole 40 throughthe oxide 24 (step 214, FIG. 3B). In illustrative embodiments, thisoxide hole 40 effectively forms an internal channel that extends to thetop surface of the upper SOI wafer.

It is expected that the oxide hole 40 first will have an inner diameterthat is substantially equal to the inner diameter of the diaphragm hole34A. A second step, such as an aqueous HF etch, may be used to enlargethe inner diameter of the oxide hole 40 to be greater than the innerdiameter of the diaphragm hole 34A. This enlarged oxide hole diameteressentially exposes a portion of the bottom side of the diaphragm 14. Inother words, at this point in the process, the channel forms an airspace between the bottom side of the diaphragm 14 and the top surface ofthe backplate 12.

Also at this point in the process, the entire photoresist layer 36 maybe removed to permit further processing. For example, the process maypattern the diaphragm 14, thus necessitating removal of the existingphotoresist layer 36 (i.e., the mask formed by the photoresist layer36). Other embodiments, however, do not remove this photoresist layer 36until step 222 (discussed below).

The process then continues to step 216, which adds more photoresist 36,to substantially fill the oxide and diaphragm holes 40 and 34 (FIG. 3C).The photoresist 36 filling the oxide hole 40 contacts the silicon of thetop SOI layer, as well as the underside of the diaphragm 14 around thediaphragm hole 34A.

The embodiment that does not remove the original mask thus applies asufficient amount of photoresist 36 in two steps (i.e., first the mask,then the additional resist to substantially fill the oxide hole 40),while the embodiment that removes the original mask applies a sufficientamount of photoresist 36 in a single step. In both embodiments, as shownin FIG. 3C, the photoresist 36 essentially acts as the single,substantially contiguous apparatus above and below the diaphragm 14.Neither embodiment patterns the photoresist 36 before the sacrificiallayer is etched (i.e., removal of the sacrificial oxide 24 andpolysilicon 26, discussed below).

In addition, the process may form the backside cavity 18 at this time.To that end, as shown in FIG. 3C, conventional processes may applyanother photoresist mask on the bottom side of the SOI wafer to etchaway a portion of the bottom SOI silicon layer. This should expose aportion of the oxide layer within the SOI wafer. A portion of theexposed oxide layer then is removed to expose the remainder of thesacrificial materials, including the sacrificial polysilicon 26.

At this point, the sacrificial materials may be removed. To that end,the process removes the sacrificial polysilicon 26 (step 218, FIG. 3D)and then the sacrificial oxide 24 (step 220, FIG. 3E). Among other ways,illustrative embodiments remove the polysilicon 26 with a dry etchprocess (e.g., using xenon difluoride) through the backside cavity 18.In addition, illustrative embodiments remove the oxide 24 with a wetetch process (e.g., by placing the apparatus in an acid bath for apredetermined amount of time). Some embodiments, however, do not removeall of the sacrificial material. For example, such embodiments may notremove portions of the oxide 24. In that case, the oxide 24 may impactcapacitance.

As shown in FIG. 3E, the photoresist 36 between the diaphragm 14 and topSOI layer supports the diaphragm 14. In other words, the photoresist 36at that location forms a pedestal 42 that supports the diaphragm 14. Asknown by those skilled in the art, the photoresist 36 is substantiallyresistant to wet etch processes (e.g., aqueous HF process, such as thosediscussed above). It nevertheless should be noted that other wet etchresistant materials may be used. Discussion of photoresist 36 thus isillustrative and not intended to limit the scope of all embodiments.

Stated another way, a portion of the photoresist 36 is within an airspace between the diaphragm 14 and the backplate 12; namely, itinterrupts or otherwise forms a part of the boundary of the air space.In addition, as shown in the figures, this photoresist 36 extends as asubstantially contiguous apparatus through the hole 34 in the diaphragm14 and on the top surface of the diaphragm 14. It is not patternedbefore removing at least a portion of the sacrificial layers. Nopatterning steps are required to effectively fabricate the microphone10.

To release the diaphragm 14, the process continues to step 222, whichremoves the photoresist 36/pedestal 42 in a single step. Among otherways, dry etch processes through the backside cavity 18 may be used toaccomplish this step. This step illustratively removes substantially allof the photoresist 36—not simply selected portions of the photoresist36.

It should be noted that a plurality of pedestals 42 may be used tominimize the risk of stiction between the backplate 12 and the diaphragm14. The number of pedestals used is a function of a number of factors,including the type of wet etch resistant material used, the size andshape of the pedestals 42, and the size, shape, and composition of thediaphragm 14. Discussion of a single pedestal 42 therefore is forillustrative purposes.

It also should be noted that various embodiments of the presentinvention is not limited to any particular shape, material, orconfiguration of the microphone 10 or diaphragm 14. The microphone 10may be, for example, round or square, solid or perforated by one or moreholes, and/or flat or corrugated. Different diaphragm configurationsmight require different or additional processes from those described.For example, additional processes may be required to form holes orcorrugations in the diaphragm 14.

Accordingly, stiction issues relating to surface tension of liquidsshould be mitigated because sacrificial layer removal processes usingliquids are completed before the pedestal 42 is removed. Moreover,removing the sacrificial material through the backside cavity 18favorably impacts microphone 10 performance by not requiring etch holesthough the diaphragm 14 (to permit communication of the etchingmaterial). Specifically, as known by those in the art, it is preferableto have a diaphragm 14 with a large area to provide a stronger, morerobust capacitive signal. Etching holes in the diaphragm 14 undesirablyreduces the effective diaphragm area, thus reducing the effectivesignal. In addition, use of the backside cavity 18 eliminates the needto pattern the photoresist 36 used to support the diaphragm 14,consequently simplifying the fabrication process.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

1. A method of forming a microphone, the method comprising: forming abackplate by etching trenches into a layer of an silicon-on-insulatorwafer, wherein the trenches have walls; lining the trenches by addingsacrificial wet etch removable material to the walls of the trenches andalong at least a top portion of the silicon-on-insulator wafer; addingsacrificial polysilicon to the lined trenches and to the sacrificial wetetch removable material on the top portion of the silicon-on-insulatorwafer; adding more sacrificial wet etch removable material toencapsulate the sacrificial polysilicon material; forming a flexiblediaphragm on at least a portion of the wet etch removable sacrificialmaterial; adding a wet etch resistant material, a portion of the wetetch resistant material being positioned between the diaphragm and thebackplate to support the diaphragm; removing the sacrificialpolysilicon; removing the sacrificial wet etch removable material beforeremoving any of the wet etch resistant material added by the act ofadding wet etch resistant material; and removing the added wet etchresistant material after removing at least part of the wet etchremovable sacrificial material.
 2. The method as defined by claim 1further comprising releasing the diaphragm, releasing comprisingremoving the sacrificial material and removing the wet etch resistantmaterial.
 3. The method as defined by claim 1 wherein the wet etchresistant material comprises a photoresist material.
 4. The method asdefined by claim 1, wherein the act of adding wet etch resistantmaterial comprises: forming a hole through the diaphragm; forming achannel through the wet etch removable sacrificial material, the channelbeing in fluid communication with the hole through the diaphragm; andadding wet etch resistant material, the wet etch resistant materialfilling the channel while the sacrificial material is removed.
 5. Themethod as defined by claim 1 wherein adding wet etch resistant materialincludes adding a contiguous apparatus of wet etch resistant material,the contiguous apparatus of wet etch resistant material being removed bythe act of removing the added wet etch resistant material.
 6. The methodas defined by claim 4, the method further comprising: etching a hole inthe sacrificial polysilicon before adding more sacrificial wet etchremovable material to encapsulate the sacrificial polysilicon material;and wherein the act of forming a channel through the wet etch removablematerial comprises forming a channel through the hole in thepolysilicon.
 7. The method as defined by claim 4, wherein the act offorming a hole through the diaphragm comprises: adding a nitride layerabove the diaphragm; exposing the diaphragm; forming a hole through thediaphragm; adding a layer of photoresist above the diaphragm; forming aresist hole in the layer of photoresist to expose the diaphragm hole. 8.The method as defined by claim 1 wherein the wet etch resistant materialis unpatterned after the portion of wet etch resistant material ispositioned between the diaphragm and the backplate.
 9. The method asdefined by claim 1 wherein the sacrificial wet etch removable materialcomprises oxide.
 10. The method as defined by claim 1 furthercomprising: forming a first hole through the diaphragm; forming a secondhole through the sacrificial material, the second hole producing achannel between a bottom surface of the diaphragm and a top surface ofthe backplate, the portion of wet etch resistant material filling thechannel.
 11. A method of forming a MEMS microphone, the methodcomprising: providing an SOI wafer having a top layer; forming asacrificial material on the top layer of the SOI wafer; forming adiaphragm on the sacrificial material; adding wet etch resistantmaterial covering the diaphragm; forming a hole through the diaphragm;forming a channel through the sacrificial material, the hole and channelbeing in fluid communication, the channel exposing a bottom surface ofthe diaphragm and a top surface of the top layer of the SOI wafer;adding wet etch resistant material having a first portion within thechannel and a second portion contiguous with the first portion that isexternal to channel and substantially completely fills the hole throughthe diaphragm, such that all of the added wet etch resistant material isa contiguous apparatus above and below the diaphragm; and removing atleast a portion of the sacrificial material before removing any of thewet etch resistant material, wherein the wet etch resistant materialcovering the diaphragm is not removed before adding wet etch resistantmaterial having a first portion within the channel.
 12. The method asdefined by claim 11 further comprising releasing the diaphragm.
 13. Themethod as defined by claim 11 further comprising removing the wet etchresistant material after removing at least part of the sacrificialmaterial.
 14. The method as defined by claim 11 wherein the SOI waferalso has a bottom layer and a middle layer between the top and bottomlayers, the method further comprising removing a portion of the bottomlayer and the middle layer to form a backside cavity.
 15. The method asdefined by claim 14 further comprising forming a backplate hole throughthe top layer of the SOI wafer.
 16. The method as defined by claim 11wherein the wet etch resistant material is unpatterned after the wetetch resistant material is added to the channel.
 17. A method of forminga MEMS microphone, the method comprising: providing a diaphragm layer ona sacrificial layer, the sacrificial layer being between the diaphragmlayer and a substrate; forming a wet etch resistant material completelycovering the diaphragm and between the substrate and the diaphragm;forming a backside cavity; and applying a wet etch material to thesacrificial material through the backside cavity to remove at least aportion of the sacrificial layer before removing any of the wet etchresistant material, at least a portion of the wet etch resistantmaterial supporting the diaphragm layer after applying the wet etchmaterial.
 18. The method as defined by claim 17 wherein the diaphragmlayer and the, sacrificial layer are fabricated on a single SOI wafer,and the substrate is a layer of the single SOI wafer.
 19. The method asdefined by claim 17 wherein providing a diaphragm layer comprisesdepositing a material on the sacrificial layer to form the diaphragm.20. The method as defined by claim 17, wherein the sacrificial layercomprises a layer of sacrificial polysilicon encapsulated in sacrificialoxide, and wherein the method further comprises removing the sacrificialpolysilicon prior to removing the sacrificial oxide.